The invention relates to a method for determining, by means of a radar apparatus, the height of a low-elevation target, the radar apparatus being provided with a transmitting and receiving unit to which are connected a transmitting and receiving antenna and a signal processing unit, whereby:
the target is illuminated by electromagnetic radiation emitted by the transmitting unit and the transmitting antenna; PA1 the transmitted signal is reflected directly via the target and indirectly via the target and the earth surface in the direction of the receiving antenna; PA1 by means of the receiving unit, complex sum signals .SIGMA. and complex elevation difference signals .DELTA. are derived from signals received by means of the receiving antenna; PA1 by means of the signal processing unit, an algorithm is carried out for deriving the height of the target from the complex sum and elevation difference signals.
The invention furthermore relates to an apparatus for determining the height of a low-elevation target, comprising a radar apparatus provided with a transmitting unit to which a transmitting antenna is connected, a receiving antenna to which a receiving unit is connected, whereby the target is illuminated by electromagnetic radiation emitted by the transmitting unit and the transmitting antenna and the transmitted signal is reflected directly by the target and by the target via the earth surface in the direction of the receiving antenna and whereby, by means of the receiving unit, complex sum signals .SIGMA. and at least complex elevation difference signals .DELTA., which are representative for the target, can be generated, a signal processor connected to the receiving unit, provided with an algorithm for determining the height of the target h.sub.t, and means connected to the signal processor for directing the transmitting antenna and the receiving antenna at an aiming point.
The height of a target can be determined by means of a monopulse radar apparatus. The monopulse principle is described in "Introduction to Radar Systems" by M. I. Skolnik, second edition, pages 160-190. A problem encountered in the determination of the height of a target which is flying at a low altitude above the sea surface and, to a lesser extent, also the earth surface, is described on pages 172-176. This problem is due to the phenomenon that the radar apparatus not only receives echo signals directly from the target, but also target echoes reflected by the sea surface. Without the occurrence of this multipath effect, the height of the target can be easily derived from the values of the target range R and the elevation error voltage E(.theta.), delivered by the monopulse radar apparatus. The multipath effect causes the elevation error voltage to assume a complex value which renders an accurate determination of the target height impossible.
The occurrence of said multipath effect can only be avoided by selecting a radar antenna beamwidth which is narrow to such an extent that only echo signals directly from the target are received, thereby excluding unwanted target echoes reflected by the sea surface. This method has the drawback, however, that with such a narrow beam, a relatively long time might be required to acquire the target. The patent U.S. Pat. No. 4,743,907 provides an elegant solution to obviate this drawback. By fully integrating two monopulse radar apparatuses, the first radar apparatus having a relatively large wavelength and a consequent wide beam, the second radar apparatus having a relatively small wavelength and a consequent narrow beam, it is possible to achieve an optimal performance in both the acquisition and tracking phases. Such a system, however, entails considerable cost.
A method for accurately determining target height, notwithstanding the occurrence of multipath, is described in the patent U.S. Pat. No. 4,769,031. According to the method disclosed in this patent, the radar antenna is not directed at the actual target, but at a point in space exactly between the actual target and its image. By subsequently measuring with at least two different radar wavelengths, which bear a predetermined relation, a system of equations can be solved, from which several possible target heights can be derived. Thus, by successively comparing possible target heights derived on the basis of this method, the target height can be determined.
An alternative method for accurately determining the target height is described in patent EP-B 0.087.355. According to this method the antenna orientation is constantly varied. Using measuring values obtained at various antenna orientations, it will again be possible to solve systems of equations, from which the target height can be derived.
Both methods are cumbersome and have the drawback that the monopulse radar apparatus is directed at an aiming point which does not coincide with the target. For a monopulse radar apparatus, this misalignment is an evident suboptimal condition which, however, in light of the state of the art is required in order to obtain a solvable system of equations.